1. Field of the Invention
The present invention relates to a cadmium free thermal diffusion zinc chloride junction formation process for making high efficiency heterojunction thin film photovoltaic cells formed from compound semiconductors, in particular, copper indium diselenide, including the gallium and sulfur alloys thereof, and to the semiconductor produced thereby.
2. Description of the Prior Art
The prior art discloses several methods for the manufacture of high light to electrical energy conversion efficiency ("efficiency") thin film photovoltaic cells formed from a first layer of copper indium diselenide in heterojunction with one or more layers of cadmium sulfide ("CdS"). In Mickelsen et al, U.S. Pat. No. 4,335,266, a method is disclosed for forming the copper indium diselenide layer in two distinct regions. The first region desirably contains an excess of copper and the second region is copper deficient. Diffusion between the two layers achieves a uniform copper indium diselenide structure in order to reduce the formation of pure copper nodules near the copper indium diselenide surface where the cadmium sulfide layer is to be deposited. Despite the Mickelsen improvements in the copper indium diselenide layer, it had still been found necessary to deposit a cadmium sulfide layer to achieve high efficiency.
While various improvements have been made in the manufacture of copper indium diselenide CdS cells, several complications remain. For example, chemical bath deposition of cadmium sulfide is used to produce the highest efficiency devices. However, this step involves a slow wet chemical step inconsistent with an otherwise inline dry fabrication process. Moreover, cadmium and thiourea are highly toxic materials which escalate manufacturing costs as a result of the handling and disposal of the hazardous wastes.
Some of the attempts to avoid the handling complications inherent in use of CdS are described in "A ZnO/p-CuInSe.sub.2 Thin Film Solar Cell Prepared Entirely by Spray Pyrolysis", M. S. Tomar and F. J. Garcia, Thin Solid Films, 90 (1982), p.p. 419-423; and "Chemical Vapor Deposited Copper Indium Diselenide Thin Film Materials Research" Final Report, March 1984, SERI/STR-211-2247. While these publications disclose copper indium diselenide/zinc oxide heterojunction formation using zinc oxide spray pyrolysis or ion beam sputtering respectively, neither method results in an efficiency of greater than 2-3%. Therefore, these publications do not disclose a commercially viable method for the replacement of CdS with zinc oxide in a thin film copper indium diselenide heterojunction cell.
Weiting, et al, U.S. Pat. No. 4,612,411, incorporated herein by reference, describes the preparation of a thin film heterojunction photovoltaic cell formed from copper indium diselenide, as a first semiconductor layer, and the formation of a two layer zinc oxide semiconductor in heterojunction with the copper indium diselenide. The first of the two zinc oxide layers comprises a relatively thin layer (100-2000 angstroms) of high resistivity zinc oxide and the second comprises a relatively thick (10,000 angstroms) zinc oxide layer having been doped to exhibit low resistivity.
Subsequently, Pollock et al, U.S. Pat. No. 5,474,939, has produced higher efficiency non-CdS cells through the application of a wet chemical deposition zinc hydroxide precipitation step. The Pollock process involves the use of a metal back contact having a first p-type semiconductor film of chemical vapor deposition ("CVD") copper indium diselenide and a second transparent n-type semiconductor film of CVD zinc oxide on the copper indium diselenide and a thin interfacial film, of transparent insulating zinc oxide, between the p-type copper indium diselenide film and the n-type zinc oxide. Pollock prepares the interfacial zinc oxide film by the chemical deposition of zinc hydroxide on the copper indium diselenide from a zinc salt solution and complexing agents comprising ammonium hydroxide or triethanolamine, thereby forming a zinc ammonium solution complex, and annealing the deposit to convert the zinc hydroxide into the zinc oxide. Thus, while Pollock uses a wet chemical deposition step of zinc hydroxide precipitate from solution in order to generate a thin interfacial zinc oxide layer, it is believed, that devices prepared by direct deposition of a zinc oxide layer on copper indium diselenide films are only 2-4% conversion efficient in spite of utilizing films capable of producing 15-17% cells.